Make it possible to mock OkHttpClient and Call.

Closes https://github.com/square/okhttp/issues/956
Merge pull request #973 from square/jwilson_0628_nonfinal

Make it possible to mock OkHttpClient and Call.
provide a friendlier API for multipart forms.

Also, escape the name and filename tokens, and include some small
changes from CR.
Use transport protocol in SPDY response.
HTTP/2 initial window size is 65535 not 65536.
Merge pull request #981 from square/adrian.http2-initial-window

HTTP/2 initial window size is 65535 not 65536
Merge pull request #980 from square/adrian.response-protocol

Use transport protocol in SPDY response
Merge pull request #949 from mlc/friendly_form_building

provide a friendlier API for multipart forms.